This invention relates to a solid state imaging apparatus having a black level correcting circuit.
Recently, there is extensively used an endoscope (scope or fiber scope) whereby an elongate insertable part is inserted into a body cavity so that organs within the body cavity may be diagnosed or inspected.
Such endoscope is used not only medically but also industrially to observe and inspect an object within a boiler, machine, pipe of a chemical plant or the like or device.
Further, there is used an electronic scope using such solid state imaging device as a charge coupled device (which shall be abbreviated as a CCD hereinafter) for an imaging means. This electronic scope has advantages that the resolution is higher than in a fiber scope, it is easy to record and reproduce picture images and such picture image processes as the magnification of picture images and the comparison of two pictures are easy. In the above mentioned electronic scope, particularly, in a color imaging system by a so-called mosaic color filter system wherein a mosaic-like color filter is secured to the image forming surface of a CCD, a photoelectric converted output is obtained in a signal type in which a color carrier is superimposed on a luminance signal obtained in a CCD. This color carrier is obtained by a mosaic color filter secured at 1 to 1 with the unit pixel of the CCD. Naturally, the sizes of the obtained luminance signal and the color carrier are proportional to the incident light amount of the CCD and, if the incident light amount is perfectly intercepted, both should be non-signals. However, even if the light is intercepted, the color carrier will not become a non-signal and a slight color carrier will be generated as a residual color carrier. When the light is intercepted, the color video signal to be finally obtained will be black but, if the above mentioned residual color carrier is present, the color video signal will not become perfect black and a colored phenomenon will be generated. This is generally called a slip of a black balance. The residual color carrier is mostly generated by the electrostatic capacity combination of the driving signal (pulse) driving the CCD with the photoelectric conversion output of the CCD. Particularly, in the electronic scope, the above mentioned CCD driving circuit is provided on the body side and the CCD driving signal is fed to the CCD provided in the endoscope tip part by using a cable of about 3 to 5 meters formed within the scope. On the other hand, the CCD photoelectrically converting output is transmitted to the body side from the tip part by also using the cable of about 3 to 5 m. As the scope is required to be as small as possible in the outside diameter for the purpose of improving the operatability, an electrostatic capacity combination will be produced between the cables and the CCD driving signal will be generated in the CCD photoelectrically converting output.
In FIG. 1, a CCD 3 is provided in the tip part of an insertable part 2 of an electronic scope 1. The output of a CCD driving circuit 6 provided within a control apparatus 4 connected to the electronic scope 1 is transmitted to the CCD 3 by a cable 7 inserted through the insertable part 2. On the other hand, the photoelectrically converted output of the CCD 3 is transmitted to a video signal processing circuit 9 provided within the control apparatus 4 by a cable 8, is processed to be a video output and is displayed on the picture surface of a monitor 10. The cables 7 and 8 are provided in parallel with one another. In fact, a shielding wire is used but it is impossible in the structure of the shielding wire to perfectly shield them. Therefore, the signals flowing through both cables 7 and 8 will interfere with each other and particularly the leak of the CCD driving signal to the photoelectrically converted output will be large.
As explained above, such phenomena as in the following will be produced in the obtained video signal by the mutual interference of signals with each other.
The greatest defect is that the black balance of the video signal will be very unstable. Generally, in a CCD, a part of the light receiving area is light-shielded by a light shielding member and the region (which shall be abbreviated as an OB region hereinafter) of the signal corresponding to this part is made a reference level of a black level. As a result of the leak of the CCD driving signal by the mutual interference into this OB region as mentioned above, the black level will become unstable.
General circuits as illustrated in FIGS. 2 to 5 will be explained hereinafter.
As shown in FIG. 2, an endoscope apparatus 11 comprises an electronic scope 12 in which an imaging means is incorporated, a light source part 13 feeding an illuminating light to this electronic scope 12 and a control apparatus 16 housing a signal processing part 14 converting the signal imaged by the electronic scope 12 to a video signal which can be displayed in a displaying apparatus.
In the above mentioned electronic scope 12, an elongate insertable part 17 is formed so as to be easy to insert into a body cavity and, on the tip surface side of this insertable part 17, an objective lens 18 and CCD 19 are arranged and an imaging means is incorporated. By the way, a color mosaic filter 20, in which filters transmitting respective color lights, for example, of red (R), green (G) and blue (B) are provided in the form of a mosaic, is pasted on the imaging surface of the CCD 19.
A light guide 21 transmitting the illuminating light is inserted through the above mentioned insertable part 17, transmits the illuminating light fed from the light source part 13 and emits it from the tip surface. This emitted illuminating light is expanded by a light distributing lens 22 and illuminates the object side.
The light source part 13 feeding the illuminating light is provided on the hand base side end surface of the above mentioned light guide 21 and is provided with a light source lamp 23 and a condenser lens 24 condensing a white light emitted from this light source lamp 2.
The image of the object illuminated with the above mentioned illuminating light is formed on the imaging surface of the CCD by the objective lens 18 and is color-separated by the color mosaic filter 20. A signal photoelectrically converted by the application of a driving pulse for transferring and reading out the object image from a CCD driving circuit 26 is read out.
The output signal of the CCD 19 is input into a double sampling circuit 27 forming the signal processing part 14. This double sampling circuit 27 double samples the CCD output signal to remove the 1/f and reset noise contained in the CCD output signal and makes it a signal improved in the S/N ratio. This signal is input into an optical black clamping circuit (which shall be abbreviated as an OB clamping circuit hereinafter) 28. In this OB clamping circuit, in order to prevent the black level from being fluctuated by the increase and decrease of the dark current of the CCD 19, the optical black period (which shall be abbreviated as the OB period hereinafter) of the input signal is direct current-clamped by an OB period clamping pulse generated from a clamping pulse and sampling pulse generating circuit 32. The output signal of this OB clamping circuit 28 is input into a cleaning circuit 29 and the OB period and H blanking period are cleaned.
The operations of the OB clamping circuit 28 and cleaning circuit 29 shall be explained in FIG. 3.
FIG. 3(b) is a graph of the output waveform of the double sampling circuit 27 and depicts a video signal having a period 101, OB period 102 and H blanking period 103. The potential 104 is a potential of the OB period 102. This potential 104 is at a level which is determined by the dark current of the CCD 19 and is also a black level. This OB period 102 is clamped by the OB clamping pulse shown in FIG. 3(a), and the dark current is removed and the black level is stabilized.
In the above mentioned cleaning circuit 29, the OB period 102 and the H blanking period 103 on which a pulsing noise 106 is superimposed in the signal from the CCD 19 are cleaned by such cleaning pulse as is shown in FIG. 3(e) generated by a clamping pulse and sampling pulse generating circuit 32.
The concrete operation of the above mentioned cleaning circuit 29 is made by switching with an analogue gate or the like the H blanking period 103 to the potential 104 in which the OB period 102 is clamped. The concrete examples of the OB clamping circuit 28 and cleaning circuit 29 are shown in FIG. 4.
In FIG. 4, the OB clamping circuit 28 is formed of a clamping coupling condenser 33 and an analogue switch. When the analogue switch is switched on within the OB period 102, this OB clamping circuit will be direct current-clamped by the clamping potential 34 and will become the potential 104. The cleaning circuit 29 is also formed of an analogue switch and is switched to the clamping potential 34 side in the cleaning pulse period 107. As a result, the potential will become the same potential as the OB clamping potential 104. Further, as there is no noise in the OB clamping potential 104, the noise superimposed in the H blanking period 103 will be perfectly removed as shown in FIG. 3(d).
The output of the above mentioned cleaning circuit 29 is input into an automatic gain controlling circuit 37 and the output of the automatic gain controlling circuit 37 is input into a .gamma. correcting circuit 38. The .gamma. correcting circuit 38 converts the .gamma. characteristic .gamma.=1 of the output video signal of the CCD 19 to .gamma.=0.45. The output of this .gamma. correcting circuit 38 is input into a low pass filter (LPF) 39, the color carrier is removed and the luminance signal Y is extracted and is input into a mixer 41.
The output of the above mentioned .gamma. correcting circuit 38 is branched. In the clamping circuits 44 and 46, by such an H blanking clamping pulse as is shown in FIG. 3(g) generated by the clamping pulse and sampling pulse generating circuit 32, the H blanking period is direct current-clamped and is input into the sample holding circuits 42 and 43. In the sample holding circuits 42 and 43, the sampling pulses generated from the clamping pulse and sampling pulse generating circuit 32 are respectively input and R and B color signals are separated and extracted from the color carrier signal and are synchronized in a synchronizing circuit 49 through LPF's 47 and 48.
The operation of the synchronizing circuit 49 shall be explained in FIG. 5.
FIG. 5(a) is a diagram of the timing of an R color signal output which is the output of the LPF 47. FIG. 5(b) is of the timing of a B color signal output which is the output of the LPF 48. These R color signal and B color signals are extracted at 1H and are input into the synchronizing circuit 49. The synchronizing circuit 49 has a 1H delay circuit, delays the R and B color signals by 1H, interpolates the parts (shown by the broken lines in the drawing) lacking the R and B color signals and obtains the synchronized signals as in FIGS. 5(c) and (d).
The synchronized R and B color signals are input into subtracters 51 and 52 and color difference signals R-Y and B-Y are produced by the luminance signal Y output from the above mentioned LPF 39 and are input into a color encoder circuit 53 in which a chroma signal is produced from the color difference signals R-Y and B-Y and is input into the above mentioned mixer 41. In this mixer, the chroma signal is superimposed on the luminance signal Y and is output as a composite signal to a monitor not illustrated.
In the above mentioned circuit in which the OB clamping circuit 28 and cleaning circuit 29 are only provided on the input side of the automatic gain controlling circuit 37, the slip of the black balance by the above mentioned interference can not be neglected, because there is a problem that, if a noise (residual carrier) is present in the OB period 102, the voltage value of the branching point 30 in FIG. 4 will fluctuate and will not be equal to the clamping potential 34.
Also, in the color difference signal generating part explained in FIG. 2, if the direct current level of the signal is fluctuated by the above mentioned fluctuation or a residual carrier is present in the OB period of the input signals of the sampling circuits 42 and 43, the black balance of the color difference signal will fluctuate when the gain of the AGC 37 varies.
Generally, in the OB period 102, the pixels of the CCD 19 are light-shielded and the number of the light-shielded pixels is limited to the required minimum. Among the horizontal direction pixels of the CCD 19, ten or more pixels are allotted as OB detecting pixels. The time of the OB period 102 is about 2 .mu.s. The OB clamping pulse is of a pulse width of about 1 .mu.s. The pulse width is so narrow that it is necessary to elevate the active frequency band of the clamping circuit. For the cleaning purpose, in order to make the pulse width of the clamping pulse as wide as possible and to make positive clamping possible with a simple circuit, the clamping period is widened and clamping is made possible by a clamping pulse, for example, of more than 1 .mu.s. However, in such a circuit in which a noise or the like is superimposed on the OB period 102 as is mentioned above, there is a problem that the black level is unstable as mentioned above.
In the publication of Japanese Patent Laid Open Application No. 236274/1987, there is disclosed a technique wherein a black level correcting means operative at a time of the third gain is provided for an output of a video amplifier at the time of no incident light. However, correction of the gain varying steplessly is not described.